An organic electroluminescence device (hereinafter, electroluminescence is occasionally abbreviated as EL) is a self-emitting device based on the principle that, when an electrical field is applied, a fluorescent material emits light using energy generated by a recombination of holes injected from an anode with electrons injected from a cathode. Organic EL devices formed from organic materials have been vigorously studied since a report on a low voltage-driven organic electroluminescence device formed by laminating layers was made by C. W. Tang et al. of Eastman Kodak Company.
There has been proposed a phosphorescent organic electroluminescence device in which an organic phosphorescent material is used in an emitting layer. Such a phosphorescent organic electroluminescence device uses excited states of the organic phosphorescent material, i.e., a singlet state and a triplet state, to provide a high luminous efficiency. When electrons and holes are recombined in an organic EL device, it is presumed that singlet excitons and triplet excitons are produced at a rate of 1:3 due to difference in spin multiplicity. Thus, a device using a phosphorescent material presumably achieves three to four times higher luminous efficiency than a device using only fluorescence.
Various studies have been made for improving the luminous efficiency of an organic electroluminescence device using a phosphorescent material.
As a result of one of such studies, there has been proposed an organic electroluminescence device in which a plurality of emitting layers are layered between the anode and the cathode (e.g., Patent Literatures 1 to 5).